The present application generally relates to the field of electric motor controls, and specifically to an automotive power steering control system.
Typically, automobiles today use power steering systems, such as electric power steering (EPS) systems. In an EPS system, a driver's effort to steer the automotive are assisted by powered systems driven electrically. For example, the EPS system provides a steering torque assist directly by an electric motor drive. The motor itself can include a three-phase permanent magnet synchronous motor, or any other motor. The motor is driven from power sources such as a vehicle battery through application of an inverter system. The drive is typically monitored and controlled by a processing unit with sensors to detect the operating conditions of the motor. The EPS drive assist is called into use based on driver demand. Specifically, as a driver applies a steering force in one direction, the EPS system supplies a further torque in the same direction. The EPS system provides damping for the road feel and rack/column effects for return and other features. In other words, the EPS system works in all four torque speed quadrants. However, in case of inverter failure, the torque from the EPS motor can generate steering torque as current conducts through shorted circuit elements and body diodes as the motor back EMF increases due to motor velocity, temporarily shorting the phases. For example, a driver could be turning while the motor drive erroneously provides damping torque resulting in higher efforts, which is undesirable.
The motor may be operated by a current mode controller. For current mode control, the current flowing into motor terminals is measured and compared to a calculated reference current signal and are representative of desired current for the motor to affect desired operating conditions.
Thus, if phase coils of a motor of the EPS system become undesirably shorted together, for example because of a MOSFET short, the motor acts as a generator device and a braking torque is applied to a shaft of the motor. (MOSFET=metal-oxide-semiconductor field-effect transistor). During this condition, vehicle operator must overcome the braking torque of the motor while turning a vehicle steering wheel in order to steer the vehicle.
Typically, EPS systems use a MOSFET failure mitigation technique for commutation while a MOSFET is shorted to produce an increased amount of assist torque in the event of a short circuit. When the MOSFET is shorted and producing braking torque, the commutation may be shut off. This is because a complementary MOSFET cannot be turned on due to a high current event. MOSFET mitigation may reduce the time and effort to steer the vehicle but may produce a large amount of torque ripple. This is due at least in part to the increased velocity when applying an assist torque. This in turn results in a harsh feeling for a driver using the steering system.
Accordingly, it is desirable to improve MOSFET fault mitigation, particularly in a steering system to provide a driver a more comfortable driving experience.